There are many applications in which stepper motors need to be referenced to a fixed position such as a zero point or home position. These include instrumentation gauges used in automotive clusters, steppers used in medical equipment and positioning steppers used in industrial controls.
Stepper motors are typically run in an open loop position mode, where there is no position feedback from the motor shaft, the system output shaft or otherwise to determine the position of the motor. A stepper motor in this configuration is assumed to be magnetically synchronous at all times. Thus, the motor position is determined by the number of commanded pulses or commanded electrical angle movements. The simple and low cost operation of the stepper motor without position feedback is ideal for many applications such as automotive instrument cluster gauges, medical or scientific measurement systems, computer peripherals such as hard disk drives and printers and many industrial applications.
Since steppers are mostly run in an open position loop, a reference point is required to determine the initial start position of the motor. This position is determined by a mechanical hard stop, a proximity sensor or other equivalent sensor. In systems that do not use a sensor, the reference position is created by methods that may compromise the accuracy of the system. For instance, the motor is continuously run into a hard stop for a fixed time period to guarantee its position near the hard stop. This method suffers from several problems. First, there is time lost to guarantee that the motor has reached the hard stop. This is because the command must be at least equal to the entire length of travel for the stepper motor. Second, there is no guarantee that the motor has rested correctly at the hard stop.
Another method for homing a stepper motor monitors the back-EMF in a non-driven phase of the motor to detect a stall condition. This method is disadvantageous because it waits for the motor to lose synchronism. Depending on the motor and gearing system, the motors rotor could kick back or become unstable at this point. Thus it might incur a position error. This back-EMF detection method could stop the motor before it loses synchronism. However, this sensing depends on a flexible plastic gearing system and a very sensitive threshold value, which is not immune to noise. In a method using a simple fixed threshold, a transient noise spike could cause a false detection point. For small motors with low back-EMF constants, stall detection requires being able to detect tens of millivolts. This requires a high signal to noise ratio in the measurement system.
Homing of a stepper motor is significant to normal operation. For instance, a speedometer must present the correct speed, where the pointer is referenced to a marking on the instruments display. An industrial labeler must place a label at the correct location. Position errors, homing sequence time and system cost and complexity of required sensors could be alleviated by an accurate and robust method for stepper referencing.